MORTAR CUBES compressive strength, asap please 1

[civilmg]

Can anyone provide me with information regarding the reliability of using 7-day breaks to gauge the 28-day compressive strength. With standard 6"x12" cylinders, about 65% of 28-day strength after 7 days of curing is reasonable. Is there a similar "rule-of-thumb" for cubes, or does the geometry of the specimen prevent any reasonable gauging of 28-day performance?

Thanks

 

[BigH]

whilst you indicated mortar - why not judge it from the requirements of 3-d, 7-d, 14-d and 28-d requirements per the grade of cement you use. This can give you an indication of the expected minimum I would suppose. Check ASTM for information on Portland Cement.

 

[Laser28]

I would be very cautious of applying the 65% estimation factor these days. I don't know where your get your cement, but the powders today are ground so finely that it is more likely that you are getting as much as 95% of the 28 day strength at 7 days. Many of these estimations are based on coarser ground cements that hydrated much more slowly than the cements of today. And of course, the addition of slag, fly ash and or silica fume will all have an effect on the rate of strength gain.

If by cubes you refer to mortar cubes that are 50mm in dimension, the cubes are usually made using a standard (Ottawa) sand whereas the cylinders use job mix aggregates. Too many variables for accurate prediction.

If you are referring to concrete cubes used in Great Britain for example, I would assume that a correlation could be established.

If you are looking for a way to reduce test specimen size, use the 100mm x 200mm cylinders - correlations do exist and have been published. The best way is to develop your own correlation curves, though.

 

[BigH]

Sorry - I misread "cubes" and thought of mortar cubes for strengths of cement - we use cubes here in India for concrete rather than cylinders and I find it similar to cylinders when comparing apples and apples. I agree with Laser28 in that the new finely ground cements does give higher strengths at 7 compared to the "older" ones. Here they use OPC33, OPC43 and OPC53 - where the # indicates the required strength at 28days of the cement cubes. For concrete, I was getting something like 85 to 90 % on one job I was monitoring (from memory) as percentage at 7-d vs 28-d.
Good comments Laser28 -

 

[Ron]

Contrary to Laser28's comments about 95% at 28 days, you are more likely to experience less than 65% at 7 days. It is common to use GGBFS or fly ash in current mix designs for cost savings. This compromises the 7-day strength gain while maintaining reasonable later term strength gain. This creates strength prediction problems based on older, established rules of thumb.

I would suggest you ask the ready mix supplier for strength gain curves for you specific mix design to help you assess the predictability.

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[Laser28]

Hey Ron - if you read my post you will see:
"And of course, the addition of slag, fly ash and or silica fume will all have an effect on the rate of strength gain."

Slagging my post will not get you points! My comment regarding rate of strength gain is based on "cement" not on "cementitious materials" or concretes containing supplementary cementing materials.

Of course, the correct answer to the original question is:
"IT DEPENDS..."

I agree with going to the source for strength gain curves, but I would contact the cement producer as well.

 

[Ron]

Laser28...no intention of slagging your post. I did read your post again and now I understand your statement better than before. Sorry for any mischaracterization of your intent, but your statement alternates between concrete strength prediction and cement strength. The intent here is prediction of concrete strength.

My point remains....get more info.

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